Subject to reduced choice (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'to reduced choice' is the wordplay.
'reduced' means to remove the last letter.
'choice' becomes 'pick' ('pick' can be a synonym of 'choice').
'pick' with its final letter removed is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject (of a talk, etc)" , "Subject (of speech, etc)" , "Matter for an argument" , "Subject (eg, of text)" , "That may be debatable" .)